This invention relates to a hologram recording and reproduction apparatus, a hologram recording and reproduction method and a hologram recording medium, and more particularly to recording and reproduction of positioning information of data when data is reproduced from a recording medium.
In recent years, a holographic technique has been and is being developed rapidly to place a holographic memory, which attracts attention as a candidate to a strong storage which competes with next-generation or second next-generation optical disks, into practical use. Thus, a hologram recording and reproduction system which makes use of the hologram technique to achieve recording and reproduction of a large amount of data has been proposed in IBM J.RES DEVELOP VOL. 44 NO. 3 May 2000 “Holographic data storage”; U.S. Pat. No. 6,281,993 issued Aug. 28, 2001 to Bernal et al. The contents of All the reference herein are incorporated by reference herein.
In the hologram storage recording and reproduction system, a coherent laser beam is split into signal light and reference light, and the signal light is strength modulated in accordance with recording data by a spatial light modulator (SLM). The modulated signal light is condensed on a hologram recording medium. Thereupon, also the reference light is illuminated upon the hologram recording medium. Consequently, the signal light and the reference light interfere with each other, and interference fringes formed as a result of the interference are recorded as a fine density pattern on the recording medium.
In order to reproduce the data recorded on the hologram recording medium, illumination light same as the reference light is introduced at the same angle as that of the reference light to the hologram recording medium. Thereupon, the data are reproduced as diffracted light corresponding to the interference fringes recorded on the hologram recording medium, and the diffracted light is condensed on an image pickup element such as a CCD image pickup element or a MOS image pickup element and is fetched as a bit pattern by the image pickup element. A received light signal obtained by the image pickup element is analyzed and reproduced as data.
Incidentally, the storage capacity of a holographic memory depends upon the volume recording density while the storage capacity of an optical disk depends upon the plane recording density. When data are recorded holographically, recording marks are not recorded directly on the hologram recording medium, but interference fringes of signal light and reference light are recorded. Therefore, it is possible also to allocate data of 1 Mbyte to one hologram spot (data page). If the volume of the hologram recording medium is utilized to repeat such multiple recording of a data page, then a large storage capacity exceeding several hundreds Gbytes can be achieved. As representative multiple systems, an angle multiple system, a shift multiple system, a wavelength multiple system, a phase modulation multiple system and so forth are available.
For recording and reproduction of such hologram storage as described above, a marker for positioning is recorded in a page same as that of a two-dimensional bit pattern. FIG. 10 shows an example of a recording data page invented by the inventor of the present patent application. Referring to FIG. 10, the data page 26 includes a bit pattern 28 representative of data and markers 24 provided at outer peripheral portions thereof for positioning the bit pattern 28 upon reproduction.
Incidentally, diffracted light which corresponds to a light beam emitted from a pixel at a certain position of a spatial light modulator (that is, reflected light of reproduction reference light from a holographic memory) is not sometimes introduced correctly to a light receiving pixel on an image pickup element set so as to receive a light beam from the pixel at the position of the spatial light modulator. In order that an original input signal may be reproduced accurately in such an instance as just described, markers 24 for positioning a reproduction bit pattern obtained from the image pickup element are inserted in the data page 26.
Accordingly, the markers 24 are detected from an image picked up by the image pickup element, and the position of the bit pattern 28 is determined based on positioning information obtained from the markers 24. Then, reading out of the bit pattern 28 is performed.
Although the positioning accuracy of the bit pattern 28 is improved by raising the recognition rate of the markers 24, in order to raise the recognition rate of the markers 24, it is necessary for the markers 24 to have a certain magnitude. Further, while, in the example of FIG. 10, four markers 24 are disposed individually at the four corners, for example, where the distortion of an image is great, it is necessary to dispose several additional markers over the overall image.
Recording of such a coded image (data page 26) produced by encoding image data 400 to be recorded as illustrated in FIG. 11 is performed in the following manner. In particular, where the shift multiplex system is used, for example, as seen in FIG. 11, data pages 26 are successively displayed on a spatial light modulator 30 to spatially optically modulate signal light. Then, interference fringes of the modulated signal light and reference light are recorded at a hologram recording spot (hereinafter referred to merely as recording spot) 32 of a hologram recording medium 34. Thereupon, the hologram recording medium 34 is moved by a very small pitch (since the hologram recording medium 34 is shown as a disk-type medium in FIG. 11, it is rotated in the direction indicated by an arrow mark so that a plurality of data pages 26 are written in an overlapping relationship at the recording spot 32 to effect shift multiplex recording.
Again, in such a data page 26 of a hologram storage as described above, while four markers 24 are disposed one by one at the four corners, for example, where the distortion of an image is great, it is necessary to dispose several additional markers over the overall image. However, if the number of markers increases, then the region to which a bit pattern 28 is to be allocated decreases, and therefore, the recording capacity per one page decreases. Further, where multiplexing of hologram recording is performed, generally there is a tendency that, as the number of multiplexed hologram records, the S/N ratio of a hologram reproduction image decreases, and a bad influence may possibly be exerted on the recognition of the markers 24. Particularly at a peripheral portion of the data page 26 as a portion at which a marker 24 is disposed, there is a tendency that also the contrast of the image decreases from an optical reason. The decrease of the contrast and the decrease of the S/N ratio cooperatively make it difficult to recognize a marker 24 with a high degree of accuracy, resulting in degradation in bit error rate.